What cette and sprocket combination should I use?

My little brother(he's 13) is getting a Mirraco Shorty for his birthday and I wanna get him a smaller sprocket and cette cause 16t and 40t is complete ****. Which combination should I use that would work? I wanna spend like 50 or 60 bucks but more or less would be fine too. The smallest ratio would be great, thanks.